Diagnostic computer tomography (CT) is used to obtain a diagnostic image of a patient's internal organs. A CT scanner forms the image using x-rays. The patient is placed in the CT scanner between an x-ray source and an x-ray sensor so that x-rays pass through the patient's body and are detected by the x-ray sensor which then generates output signals based on the x-rays. A computer receives the signals and processes the signals to form parts of an image. The computer assembles the partial images from the scan to generate a desired cross section image of the body, including the organ or organs targeted for diagnosis.
A diagnostic CT examination commonly targets a specific body organ for diagnosis. For example, a thoracic CT scan typically obtains an image of the lungs or heart. Other organs commonly targeted for CT scans include the brain, spine and pelvis. The CT scan should return to the doctor a readable diagnostic image of the target organ.
During. a CT scan, an organ other than the target organ may be exposed to x-rays. X-rays pass through the entire cross section of the body, and the x-rays will pass through an organ adjacent the target organ. The adjacent, secondary organ is seldom the target of a diagnostic CT examination. The secondary organ is exposed to x-rays as an incidental by-product of the CT examination of the target organ.
An organ may be radiosensitive, that is, the organ may be susceptible to cancer induced by radiation (including x-rays). Common radiosensitive organs exposed to x-rays as an incidental by-product of a CT examination include the breasts, eyes, thyroid and scrotum.
Incidental exposure of a radiosensitive secondary organ to x-rays may increase the risk of cancer. For example, a thoracic CT examination typically exposes each breast to a radiation dose of 2.0-3.5 rads. Young women are especially sensitive to radiation-induced breast cancer. It is desirable to reduce incidental exposure of secondary organs to x-rays.
Thus, there is a need for an improved method of obtaining a diagnostic image using a CT scanner. The method should reduce x-ray exposure to a radiosensitive secondary organ during CT examination of a target organ. The method should not degrade the diagnostic portion of the image nor contain artifacts that extend into the diagnostic portion of the diagnostic image. The method should be usable with existing CT scanners and scanner protocols.